Circuit board, multi-layer wiring board method for making circuity board, and method for making multi-layer wiring board

ABSTRACT

The object of the present invention is to provide a multilayer flexible wiring board which is capable of effecting interlaminar connection with certainty, has high reliability and allows lamination of outer layer wiring boards. According to the present invention, there is provided a multilayer flexible wiring board including (1) plural single-sided wiring boards each having a wiring pattern on one side of the corresponding substrate and two-layer conductor posts projecting from the wiring pattern to the side of the substrate opposite from the wiring pattern, wherein the substrates other than that of the outermost layer have the pads to be connected to the two-layer conductor posts on the side opposite from the conductor posts, and the wiring pattern has no surface coating; (2) a flexible wiring board having on at least one side thereof the pads for connection to the conductor posts and including a wiring pattern with surface coating applied on the flexible portion but no surface coating applied on the multilayer portion, and (3) an,adhesive layer having a flux function, wherein the conductor posts and pads are connected by a metal or an alloy, and the wiring patterns are electrically connected.

TECHNICAL FIELD

The present invention relates to circuit boards, multilayer wiringboards, a method of producing the circuit boards, and a method ofproducing the multilayer wiring boards. More particularly, it relates tomultilayer flexible printed wiring boards used as a part of electronicequipment, circuit boards forming a component of such multilayer wiringboards, and methods of producing these boards.

BACKGROUND ART

With enhancement of packaging density of electronic equipment in recentyears, progress has also been made in layer multiplication of theprinted wiring boards used in such electronic equipment and use of theflexible wiring boards of a multilayer structure is gaining ground. Theprinted wiring board is a rigid-flexible wiring board which is acomposite of a flexible wiring board and a rigid wiring board, and therange of its use is expanding.

The following methods have been proposed for producing the multilayerflexible wiring boards or rigid-flexible wiring boards: The patternedcopper foils and the insulating layers are placed one on the otheralternately to form a laminate, then the through-holes for interlaminarconnection are formed in the said laminate, and after applying platingfor interlaminar connection on the said through-holes, working of theoutermost layer circuits, etc., is conducted; On the insulator side ofeach single-sided wiring board are formed the holes which do notpenetrate the copper foil, then conductor posts are formed with a metalor an alloy, followed by surface coating of the whole layers, and anadhesive layer and each wiring board are press bonded, with the aboveoperations being repeated a necessary number of times for effectuatingthe desired layer multiplication. (See, for example, JP-A-11-54934.)

In the former method, interlaminar connection is made by forming thethrough-holes in such a way that they pierce through all the layers.However, this connecting method, although simple in working, is subjectto many restrictions on circuit designing. The greatest drawback of thismethod is that since all the layers are connected by through-holes, thethrough-holes formed in the outermost layers are increased in number andalso the areal ratio occupied by the through-hole lands elevates, makingit unable to raise the circuit density which is essential for densepackaging of parts and circuit patterning. Also, high-density packagingand patterning, for which the market demand is expected to rise in thefuture, would become works of more complex specifications. With furtheradvance of miniaturization and densification of the packaged parts, thelayer connecting lands and through-holes need to be formed at a samelocation through the layers, so that the wiring density becomesdeficient in design, giving rise to the problems in packaging of theparts.

In the conventional production methods of the flexible wiring boards, inorder to reduce the production cost, they are manufactured by arrangingplural patterns on a single sheet. It is possible to produce themultilayer flexible wiring boards, too, at low cost by using the samemethod. According to this method, however, if any defective patternexists in the sheet, the multilayer flexible wiring board having such adefective pattern in the laminate also become defective, causing areduction of process yield in the laminating process. The greatestdifference between the multilayer flexible or rigid-flexible wiringboard and the multilayer rigid wiring board is that the former has aflexible portion while the latter does not. In construction of thisflexible portion, it needs to eliminate the outer layers so that theflexible portion won't be laminated or to remove the outer layers afterlamination, so that a drop of material yield is inevitable in sheetlamination. Further, in the case of designing the different sizes ofpatterns for the respective layers, the number of the patterns that canbe formed per sheet is restricted by the number of the maximum sizepatterns, resulting in an unsatisfactory pattering ratio and a lowmaterial yield.

In the latter production method, there are included the steps ofspecific operations for boring the substrate on the conductor postreceiving side by laser, applying a desmearing treatment and formingopenings in the surface coating, so that there exist the problems on theestablishment of techniques for these specific operations and reductionof yield. There also is the problem that as the number of the structurallayers increases, a longer time and higher cost are necessitated for theproduction and further the material cost for the surface coatingelevates.

DISCLOSURE OF INVENTION

The present invention, in order to eliminate the above problems,envisions to provide the multilayer flexible wiring boards which areeasy to manufacture and capable of making interlaminar connection withcertainty, have high reliability and allow lamination of the outer layerwiring-boards, and a method of producing such multilayer flexible wiringboards.

The above object of the present invention can be fulfilled byimplementing the following embodiments (1) to (22) of the invention.

(1) A circuit board comprising:

an insulated substrate;

a conductor circuit formed on one side of said substrate; and

two-layer conductor posts electrically connected to said conductorcircuit;

wherein each of said two-layer conductor posts is formed in a holepiercing said insulated substrate and comprises a projecting terminalhaving its one end connected to said two-layer conductor circuit and itsother end projecting from the other side of said insulated substrate,and a metal coating layer covering the portion of said terminal thatprojects out from the other side of said insulated substrate.

(2) A circuit board as set forth in (1) wherein the metal coating layeris composed of at least one metal selected from the group consisting ofgold, silver, nickel, tin, lead, zinc, bismuth, antimony and copper, oran alloy containing such metals.

(3) A circuit board comprising:

an insulated substrate;

a conductor circuit formed on one side of said insulated substrate; and

two-layer conductor posts electrically connected to said conductorcircuit;

wherein an adhesive layer having a function of flux is provided on oneor both sides of said insulated substrate.

(4) A circuit board comprising:

an insulated substrate;

a conductor circuit formed on one side of said insulated substrate; and

two-layer conductor posts electrically connected to said conductorcircuit;

wherein a surface coating is provided on one side of said insulatedsubstrate, said coating covering said conductor circuit with a partthereof left uncovered, and an adhesive layer having a flux function isprovided on the other side of said insulated substrate.

(5) A circuit board as set forth in any one of (1) to (4) wherein saidtwo-layer conductor posts contain copper and a metal or copper and analloy.

(6) A circuit board as set forth in (3) or (4) wherein each of saidtwo-layer conductor posts is formed in a hole piercing said insulatedsubstrate, and comprises a protruding terminal of which one end isconnected to said conductor circuit and the other end projects from theother side of said insulated substrate, and a metal coating layercovering the portion of said protruding terminal which projects from theother side of said insulated substrate.

(7) A circuit board as set forth in (6) wherein said metal coating layeris made of at least one metal selected from the group consisting ofgold, silver, nickel, tin, lead, zinc, bismuth, antimony and copper, oran alloy containing such metals.

(8) A multilayer wiring board comprising a laminate of plural circuitboards including the one set forth in (1) or (2).

(9) A multilayer wiring board comprising a laminate of plural circuitboards including the one set forth in (3) or (4).

(10) A multilayer wiring board comprising a laminate of plural circuitboards including the one set forth in any one of (1) to (4) and acircuit board comprising:

an insulated substrate;

a conductor circuit formed on both sides of said insulated substrate;

a metallic layer formed covering a part of said conductor circuit; and

a surface coating covering the portion of said conductor circuit otherthan said metallic layer.

(11) A multilayer wiring board comprising a laminate of plural circuitboards including the one set forth in (1) or (2), the one set forth in(3) or (4), and a circuit board comprising:

an insulated substrate;

a conductor circuit formed on both sides of said insulated substrate;

a metallic layer formed covering a part of said conductor circuit; and

a surface coating covering the portion of said conductor circuit otherthan said metallic layer.

(12) A multilayer wiring board in which the circuit board set forth inany one of (1) to (4) is joined to both sides of another circuit boardset forth below, and the conductor circuits of the respective circuitboards are electrically connected at the specified sites through saidconductor posts, said another circuit board comprising:

an insulated substrate;

a conductor circuit formed on both sides of said insulated substrate;

a metallic layer formed covering a part of said conductor circuit; and

a surface coating covering the portion of said conductor circuit otherthan said metallic layer.

(13) A multilayer wiring board in which a circuit board set forth in (3)or (4) is joined to both sides of another circuit board set forth below,a circuit board set forth in (1) or (2) is joined to said both circuitboards, and the conductor circuits of the respective circuit boards areelectrically connected at the specified positions through said conductorposts, said another circuit board comprising:

an insulated substrate;

a conductor circuit formed on both sides of said insulated substrate;

a metallic layer formed covering a part of said conductor circuit; and

a surface coat covering the portion of said conductor circuit other thansaid metallic layer.

(14) A multilayer wiring board as set forth in any one of (11) to (13)wherein said surface coating includes an adhesive layer.

(15) A multilayer wiring board as set forth in (7) having a multilayerportion comprising a laminate of plural circuit boards, and asingle-layer portion to which at least one circuit board in saidmultilayer portion extends therefrom.

(16) A multilayer wiring board as set forth in (15) wherein the circuitboard constituting said single-layer portion is a flexible circuitboard.

(17) A multilayer flexible wiring board comprising (i) pluralsingle-sided wiring boards having a wiring pattern formed on one side ofa substrate made of an insulating material and two-layer conductor postsmade of copper and a metal or copper and an alloy, each said conductorpost projecting from said wiring pattern to the side of said substrateopposite from said wiring pattern, with the substrates other than thatof the outermost layer having, on the side opposite from said conductorposts, the pads for making connection to the conductor posts, and saidwiring pattern having no surface coating, (ii) a flexible wiring boardhaving on at least one side thereof the pads for connection to saidconductor posts and comprising a wiring pattern with surface coatingapplied on the flexible portion but no surface coating applied on themultilayer portion, and (iii) an adhesive layer having a flux functionwhereby the respective boards are laminated integrally, wherein saidconductor posts and pads are connected by a metal or an alloy throughthe medium of said adhesive layer, and said wiring patterns areelectrically connected.

(18) A multilayer flexible wiring board as set forth in (17) whereinsaid flexible wiring board is a severed individual piece.

(19) A multilayer flexible printed wiring board as set forth in (17) or(18) wherein the metal is at least one of gold, silver, nickel, tin,lead, zinc, bismuth, antimony and copper.

(20) A multilayer flexible printed wiring board as set forth in any oneof (17) to (19) wherein the alloy comprises at least two of tin, lead,silver, zinc, bismuth, antimony and copper.

(21) A method of producing a multilayer flexible wiring board comprisingthe steps of: boring a substrate made of an insulating material, andthen forming on the bored side of said substrate the protrudingtwo-layer conductor posts made of copper and a metal or copper and analloy; forming a wiring pattern on the side of said substrate oppositefrom said two-layer conductor posts; forming an adhesive layer having aflux function over the whole surface of each of the substrates otherthan that of the outermost layer on the wiring pattern side having thepads opposite from said two-layer conductor post side by lamination orprinting, thereby forming a single-sided wiring board; forming aflexible wiring board comprising a wiring pattern having on at least oneside thereof the pads for joining to said two-layer conductor posts;forming an adhesive layer having a flux function on the wiring patternside having the pads of said flexible wiring board over the wholesurface or partially thereof by lamination or printing; and heat-pressbonding said two-layer conductor posts and said pads through the mediumof said adhesive layer.

(22) Multilayer flexible wiring boards that can be obtained from themethod set forth in (21).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) to 1(g) are the sectional views for illustrating asingle-sided wiring board that forms the outermost layer used in thepresent invention and a process for producing it.

FIGS. 2(a) to 2(e) are the sectional views for illustrating a flexiblewiring board used in the present invention and a process for producingit.

FIGS. 3(a) and 3(b) are the sectional views for illustrating amultilayer flexible wiring board of a four-layer structure according tothe present invention and a process for producing it.

FIGS. 4(a) to 4(g) are the sectional views for illustrating asingle-sided wiring board that forms the inner layers used in thepresent invention and a process for producing it.

FIGS. 5(a) and 5(b) are the sectional views for illustrating amultilayer flexible wiring board of a six-layer structure according tothe present invention and a process for producing it.

The reference numerals used in the drawings designate the following:

-   101, 201, 401: copper foil-   102, 202, 402: substrate-   205, 406: wiring pattern-   107, 206: surface coating-   108: opening in surface coating-   103, 403: opening in substrate-   105, 405: two-layer conductor post-   104, 404: copper post-   110: single-sided laminate-   106, 204: pad-   207, 407: adhesive layer having a function as a flux-   120: outer layer single-sided wiring board-   203: through-hole-   210: double-sided board-   220: interlayer flexible wiring board-   310: multilayer (four-layer) flexible wiring board-   320, 520: multilayer portion-   330, 530: flexible portion-   410: single-sided laminate-   420: interlayer single-sided wiring board-   510: multilayer (six-layer) flexible wiring board

EMBODIMENTS OF THE INVENTION

The present invention will be described regarding the embodimentsthereof with reference to the accompanying drawings, but it should beunderstood that these embodiments are merely intended to be illustrativeand not to be construed as limiting the scope of the invention in anyway.

FIGS. 1 to 5 are the drawings illustrating the examples of themultilayer flexible wiring boards embodying the present invention andthe manufacturing methods thereof. FIG. 3(b) and FIG. 5(b) are thesectional views showing the structure of the multilayer flexible wiringboards obtained according to the present invention, with FIG. 3(b)showing a four-layer flexible wiring board 310 having a multilayerportion 320 and a flexible portion 330, and FIG. 5(b) showing asix-layer flexible wiring board 510 having a multilayer portion 520 anda flexible portion 530.

For explaining the producing methods of the multilayer flexible wiringboards according to the present invention, first a process for producingan exemplary four-layer flexible wiring board is illustrated. Theprocess can be divided into the following three steps A, B and C. In thestep A (FIG. 1), the outer layer single-sided wiring boards 120 areformed. Then, in the step B (FIG. 2), an interlayer flexible wiringboard 220 is formed, and finally in the step C (FIG. 3), the outer layersingle-sided wiring boards 120 are laminated on the interlayer flexiblewiring board 220 to complete a multilayer flexible wiring board 310.

For producing a wiring board with five or more layers, the single-sidedwiring boards 120 made in the step A are used as the outermost layerwiring boards. For making the structure from the second to the centerlayer both-sided boards from the outside, the interlayer single-sidedwiring boards 420 are formed in the step D (FIG. 4), with the interlayerflexible wiring board 220 formed in the step B being used as the centerlayer wiring board. In the step E, the interlayer single-sided wiringboards 420 are laminated on the interlayer flexible wiring board 220which forms the center layer, and the single-sided wiring boards 120 arelaminated thereon as the outermost layers to form a multilayer flexiblewiring board 510. For producing a structure with five or more layers, adesired number of interlayer single-sided wiring boards 420 arelaminated between each outermost layer single-sided wiring board 120 andthe interlayer flexible wiring board 220 composing the center layer.

For making the outer layer single-sided wiring board 120 in the step A,there is first prepared a single-sided laminate 110 having a copper foil101 deposited on one side of a substrate 102 comprising an insulatingmaterial obtained by curing a resin such as a polyimide or epoxy resin(FIG. 1(a)). It is to be noted here that for preventing smearing whichis detrimental to conductor connection, preferably no adhesive layer isallowed to exist between the substrate and the copper foil for bondingthem, although use of an adhesive for bonding is not alwaysunacceptable. In the substrate 102, openings 103 are formed till thecopper foil 101 is exposed (FIG. 1(b)). Such openings can be easilyformed by the laser method which also allows formation of holes of smalldiameters. It is also preferable to remove the resin remaining in thesubstrate openings 103 by such a method as dry desmearing using anaqueous solution of potassium permanganate or plasma dry desmearing asthis contributes to the enhancement of reliability of interlaminarconnection. In each of these substrate openings 103, a two-layerconductor post 105 is formed projecting from the substrate 102 surface(FIG. 1(d)). The two-layer conductor post 104 can be formed by firstforming a copper post 104 by a pasting or plating method (FIG. 1(c)) andthen coating it with a metal or an alloy. As the metal, at least one ofgold, silver, nickel, tin, lead, zinc, bismuth and antimony is used, andthe post may be single layered or may have a structure of two or morelayers. The alloy used here is a solder comprising two or more metalsselected from tin, lead, silver, zinc, bismuth, antimony and copper. Forinstance, tin-lead, tin-silver, tin-zinc, tin-bismuth, tin-antimony,tin-silver-bismuth and tin-copper alloys can be used, but the solderused in the present invention is not restricted by the metal combinationor composition and any optimal alloy can be selected for use. Thicknessof the post is 0.05 μm or greater, preferably 0.5 μm or greater. Thenthe copper foil 101 on one side of the substrate 102 is etched to form awiring pattern 106 (FIG. 1(e)), and a surface coating 107 is applied onthe wiring pattern (FIG. 1(f)). This surface coating 107 can be formedby pasting an overlay film comprising an adhesive-applied insulatingresin or by printing an ink directly on the substrate. In this surfacecoating 107 may be formed openings 108 for facilitating surfacetreatment-such as plating.

An adhesive layer having a flux function may be formed on the side ofthe substrate 102 where the two-layer conductor posts 105 project, butit is preferable for preventing oxidation of the wiring pattern 204(FIG. 2(c)) to form the said adhesive layer on the interlayer flexiblewiring board 220 (FIG. 2(e)) having the pads for connection to theconductor posts. The said adhesive layer having a flux function can beformed by various methods such as applying an adhesive having a fluxfunction on the substrate 102 by a printing method, but it is moreexpedient to laminate a sheet of adhesive on the substrate 102. Finally,the work is properly cut according to the size of the multilayer portionto obtain an individual piece of outer layer single-sided wiring board120 (FIG. 1(g)).

This outer layer single-sided wiring board 120 may also be obtained byfirst forming a wiring pattern 106 on a single-sided laminate 110, thenforming the openings 103 in the substrate, and providing the two-layerconductor posts 105 and surface coating 107.

The adhesive having a flux function used in the present invention is anadhesive having a function to clean the metal surface, for example, afunction to remove or reduce the oxide film present on the metalsurface, and a primarily preferred adhesive composition comprises (A) aresin such as phenolic novolak resin having a phenolic hydroxyl group,cresol novolak resin, alkylphenolic novolak resin, cresol resin andpolyvinylphenol resin, and (B) a curing agent for these resins. Examplesof the curing agents usable here include phenol-based epoxy resins suchas bisphenol-based, phenolic novolak-based, alkyklphenolicnovolak-based, biphenol-based, naphthol-based and resorcinol-based ones,the epoxy resins epoxidized with an aliphatic, alicyclic or unsaturatedaliphatic skeleton as base, and the isocyanate compounds.

The amount to be blended of a resin having a phenolic hydroxyl group ispreferably 20 to 80% by weight in the whole adhesive. If its amount isless than 20% by weight, the activity of the composition to clean themetal surface is low, while if its amount exceeds 80% by weight, it ishardly possible to obtain a sufficiently cured product, resulting in areduced bonding strength and reliability. On the other hand, the ratioof the resin or compound acting as a curing agent is preferably 20 to80% by weight in the whole adhesive. The adhesive may contain wherenecessary various additives such as coloring matter, inorganic filler,various kinds of coupling agent, solvent, etc.

The secondarily preferred adhesive composition comprises (C) aphenol-based epoxy resin such as bisphenol-based, phenol novolak-based,alkylphenol novolak-based, biphenol-based, naphthol-based andresorcinol-based ones, or an epoxy resin epoxidized with an aliphatic,alicyclic or unsaturated aliphatic skeleton as base, and (D) a curingagent for said epoxy resins which has an imidazole ring. Examples of thecuring agents having an imidazole ring are imidazole, 2-methylimidazole,2-ethyl-4-methylimidazole, 2-phenylimidazole,1-benzyl-2-methylimidazole, 2-undecylimidazole,2-phenyl-4-methylimidazole, and bis(2-ethyl-4-methyl-imidazole).

The epoxy resin content is preferably 30 to 99% by weight in the wholeadhesive. If its content is less than 30% by weight, a satisfactorycured product may not be obtained. The adhesive may contain, beside thesaid two components, a thermosetting or thermoplastic resin such ascyanate resin, acrylic resin, methacrylic resin and maleimide resin. Ifnecessary, a coloring matter, inorganic filler, various kinds ofcoupling agent, solvent and such may also be added. The resin having animidazole ring and acting as a curing agent for said epoxy resins iscontained in an amount of 1 to 10% by weight in the whole adhesive. Ifits content is less than 1% by weight, the metal surface cleaningperformance of the adhesive is unsatisfactory and the epoxy resin maynot be sufficiently cured. If its content exceeds 10% by weight, thecuring reaction may advance rapidly, causing deterioration of fluidityof the adhesive layer.

Various methods are available for preparing the adhesive. For instance,a solid resin (A) having a phenolic hydroxyl group and a solid resin (B)acting as a curing agent are dissolved in a solvent; a solid resin (A)having a phenolic hydroxyl group is dissolved in a liquid resin (B)acting as a curing agent; a solid resin (B) acting as a curing agent isdissolved in a liquid resin (A) having a phenolic hydroxyl group; or aresin (D) having an imidazole ring and acting as an epoxy resin curingagent is dispersed or dissolved in a solution prepared by dissolving asolid epoxy resin (C) in a solvent. As the solvent, acetone, methylethyl ketone, methyl isobutyl ketone, cyclohexane, toluene, butylcellosolve, ethyl cellosolve, N-methylpyrrolidone, y-butylactone and thelike can be used. A solvent having a boiling point of 200° C. or belowis preferably used.

A process for making an interlayer flexible wiring board 220 in the stepB comprises the following procedure. A double-sided board 210 comprisinga heat-resistant resin 202 commonly used for the flexible wiring boardssuch as a polyimide and copper foils 201 is prepared (FIG. 2(a)). Thisdouble-sided board 210 constitutes the flexible portion of the productmultilayer flexible wiring board. Preferably no adhesive layer isallowed to exist between the copper foils 201 and the heat-resistantresin 202 for enhancing the flexing properties although the presence ofsuch an adhesive layer is not always ruled out. The front and back sidesof this double-sided board 210 are made electrically conductive by thethrough-holes 203 (FIG. 2(b)), and then the wiring pattern and the pads204 capable of receiving the two-layer conductor posts 105 are formed byetching (FIG. 2(c)). After that, a surface coating 206 of a polyimide orthe like is applied on the part of the wiring pattern 205 thatcorresponds to the flexible portion 330 (FIG. 2(d)) to for an interlayerflexible wiring board. Here, in order to improve wettability with thetwo-layer conductor post 105 to secure connection reliability, thewiring pattern 204 may be subjected to a surface treatment by plating orsolder pasting with a metal or an alloy. The metal to be used here isnot specified, but tin is preferred because of low melting point. Thealloy used here is a solder comprising at least two metals selected fromtin, lead, silver, zinc, bismuth, antimony and copper. For example, thecombinations of tin/lead, tin/silver, tin/zinc, tin/bismuth,tin/antimony, tin/silver/bismuth, and tin/copper are suggested, but thesolder usable in this invention is not restricted by the metalcombination or composition and the best suited solder is selected andused. Then, an adhesive layer 207 having a flux function is formed onthe part of the wiring pattern 204 corresponding to the multilayerportion 320 (FIG. 2(e)). The interlayer flexible wiring board may be cutinto an individual piece before lamination.

Formation of a multilayer flexible wiring board 310 in the step C isaccomplished by laying up an individual piece of outer layersingle-sided wiring board 120 on both sides of an interlayer flexiblewiring board 220. Positioning in this operation can be effected by amethod in which the positioning mark previously formed on the wiringpattern of each layer is read by an image recognizer, or a method inwhich positioning is conducted using the positioning pins. Then the workis heated to a temperature at which soldering is possible, and thenpressed under heating until the two-layer conductor posts 105 and thepad portions 204 of the interlayer flexible wiring board 220 aresoldered together with the flux-functioning adhesive layer 207interposed between them, after which the work is reheated at atemperature which does not cause melting of the solder to cure theadhesive layer 207 and effect interlaminar adhesion, therebyeffectuating lamination of the outer layer single-sided wiring boards120 and the interlayer flexible wiring board 220 (FIG. 3(b)). Laminationof the respective layers can be accomplished, for instance, by vacuumpressing or a combination of hot lamination and baking.

In the foregoing, the structure of a multilayer flexible wiring boardhaving its multilayer portion comprised of four layers has beendescribed with reference to FIGS. 1 to 3, but the embodiments of thepresent invention include a two-layer structure in which the pads areprovided on one side alone of an interlayer flexible wiring board, withone individual piece of outer layer single-sided wiring board being laidon each of the said pads, and a three-layer structure in which theinterlayer flexible wiring board is double-sided. The four or morelayered flexible wiring boards having the individual pieces of one-sidedwiring board laid up successively are also included in the embodimentsof the present invention. In the following, a multilayer flexible wiringboard with its multilayer portion composed of six layers is explained.

In the case of the six-layer structure, the one-sided wiring boards 120forming the outermost layers and the interlayer flexible wiring board220 forming the center layer are the same as those used in thefour-layer structure, but an interlayer single-sided wiring board 420 isformed as the layer disposed between each outermost layer and the centerlayer in the step D (FIG. 4).

A process for making the interlayer single-sided wiring board 420 in thestep D comprises the following procedure. A single-sided laminate 410 isprepared by attaching a copper foil 401 on one side of a substrate 402made of an insulating material obtained by curing a resin such aspolyimide or epoxy resin (FIG. 4(a)). In this operation, in order toprevent smearing which becomes an obstacle to conductor connection, itis preferable not to allow presence of an adhesive layer between thesubstrate and the copper foil for bonding them although use of anadhesive for bonding is possible in the concept of the presentinvention. Then openings 403 are formed in the substrate 402 to theextent that the copper foil 401 is exposed (FIG. 4(b)). Such openingscan be easily formed by the laser method which also allows formation ofholes of small diameters. It is also preferable to remove the resinremaining in the substrate openings 403 by such a method as drydesmearing using an aqueous solution of potassium permanganate or plasmadry desmearing as this contributes to the enhancement of reliability ofinterlaminar connection. In each of these substrate openings 403, atwo-layer conductor post 405 is formed projecting from the substrate 402surface (FIG. 4(d)). The two-layer conductor post 405 can be formed byfirst forming a copper post 404 by a pasting or plating method (FIG.4(c)) and then coating it with a metal or an alloy. As the metal, atleast one of gold, silver, nickel, tin, lead, zinc, bismuth and antimonyis used, and the post may be either single layered or may have astructure of two or more layers. The alloy used here is a soldercomposed of two or more metals selected from tin, lead, silver, zinc,bismuth, antimony and copper. For instance, tin-lead, tin-silver,tin-zinc, tin-bismuth, tin-antimony, tin-silver-bismuth and tin-copperalloys can be used, but the solder used in the present invention is notrestricted by the metal combination or composition and any optimal alloycan be selected for use. Thickness of the post is 0.05 μm or greater,preferably 0.5 μm or greater. Then the copper foil 401 on one side ofthe substrate 402 is etched to form a wiring pattern 406 (FIG. 4(e)),and an adhesive layer 407 having a flux function is formed on thiswiring pattern 406 (FIG. 4(g)). This adhesive layer having a fluxfunction can be formed by printing, but it is more expedient to laminatea sheet of adhesive on the wiring pattern 406. This adhesive layer maybe formed where necessary on the side having the pads for connection tothe posts. Interposition of one such adhesive layer can secureconnection between laminations. Finally, the work is properly cutaccording to the size of the multilayer portion to obtain an individualpiece of outer layer single-sided wiring board 420 (FIG. 4(g)).

For forming a multilayer flexible wiring board 510 in the step E, theinterlayer single-sided wiring boards 420 are laid up on the interlayerflexible wiring board 220, and the outermost layer single-sided wiringboards 120 are further laid up on the outside thereof.

The structures of seven or more layers can be obtained by laminating arequired number of interlayer single-sided wiring boards 420.

The hot press bonding method for layer multiplication is notspecifically defined in the present invention. For example, hot pressbonding may be conducted each time the individual piece of interlayerflexible wiring board is laid up. Alternatively, all the individualpieces of outer surface single-sided wiring board may be laid up first,followed by hot press bonding of the whole laminated boards. As anothermethod, the pads to be connected to the two-layer posts are soldered byapplying heat exceeding the fusion point of the solder in the step oftentative bonding of the lay-up, and then the interlaminar adhesive iscured at a temperature below the fusion point to provide the desiredlaminate.

EXAMPLE 1

(Production of Outer Layer Single-Sided Wiring Board)

A single-sided laminate 110 was formed by affixing a 12 μm thick copperfoil 101 on one side of a 50 μm thick substrate 102 (SUMILITE APL-4001produced by Sumitomo Bakelite Co., Ltd.) made of an insulating materialcomprising a cured epoxy resin. UV laser beams were applied to thislaminate 110 from the substrate 102 side to form openings 103 with adiameter of 100 μm in the substrate 102, and they were subjected to adesmearing treatment with an aqueous solution of potassium permanganate.Then copper electroplating was applied to the inside of the substrateopenings 103 to a build-up of 55 μm, followed by 5 μm thick solderplating to form the two-layer conductor posts 105. Then, the copper foil101 of the single-sided laminate 110 was etched to form a wiring pattern106, and after printing a liquid resist (SR9000W produced by HitachiChemical Co., Ltd.), a surface coating 107 was applied. Finally, thework was shaped to the regulated size of the laminate portion to obtainan outer layer single-sided wiring board 120.

(Production of Interlayer Flexible Wiring Board)

A two-layer double-sided board 210 (NEX23FE (25T) produced by MitsuiChemical Co., Ltd.) consisting of the 12 μm thick copper foils 201 and a25 μm thick polyimide film was bored by a drill, and after applyingdirect plating, through-holes 203 were formed by copper electroplatingto make the front and back sides of the board electrically conductive.Then a wiring pattern and the pads 204 capable of receiving thetwo-layer conductor posts 105 were formed by etching. Thereafter, asurface coating 206 was formed on the part of the wiring pattern 205corresponding to the flexible portion 330 by applying thereto a 25 μmthick polyimide (APICAL NPI produced by Kanegafuchi Chemical IndustryCo., Ltd.) and a 25 μm thick heat-hardening adhesive (a materialdeveloped by our company). Further, a 20 μm thick heat-hardeningadhesive sheet having a flux function (Interlaminar Adhesive Sheet RCFproduced by Sumitomo Bakelite Co., Ltd.) was laminated on the part ofthe wiring pattern 204 corresponding to the multilayer portion 320 toform an adhesive layer 207 having a flux function, thereby constitutinga sheet-sided interlayer flexible wiring board 220.

(Production of Multilayer Flexible Wiring Board)

The outer layer single-sided wiring boards 120 were laid up on bothsides of the interlayer flexible wiring board 220 by using a fixturehaving the positioning pin guides. Then, after tentative bonding by avacuum press laminator at 130° C. under 0.2 MPa for 60 seconds, the workwas pressed by a hydraulic press at 260° C. under 0.02 MPa for 30seconds. The two-layer conductor posts 105 were soldered to the pads 204of the interlayer flexible wiring board 220 with the interposition ofthe adhesive layer 207 having a flux function to form a metal bond, andthen the assembly was heated at 150° C. for 60 minutes to obtain alayer-built (multilayer) flexible wiring board 310.

EXAMPLE 2

Multilayer flexible wiring boards were obtained in the same way as inExample 1 except that in producing the outer layer single-sided wiringboards, the two-layer conductor posts 105 were formed by changing thediameter of the substrate openings 103 down to 50 μm at the smallest.

EXAMPLE 3

A multilayer flexible wiring board was obtained in the same way as inExample 1 except that solder plating was applied on the part of thewiring pattern 204 corresponding to the multilayer portion 320 of theinterlayer flexible wiring board.

COMPARATIVE EXAMPLE 1

A multilayer flexible wiring board was obtained in the same way as inExample 1 except that a surface coating 206 was formed on the entiretyof the wiring pattern of the interlayer flexible wiring board 220, thatthe openings in the surface coating serving as the pads for receivingthe conductor post 105 were formed by CO₂ laser, and that a step ofapplying a desmearing treatment was added.

COMPARATIVE EXAMPLE 2

A multilayer flexible wiring board was obtained in the same way as inExample 1 except that the adhesive sheet 207 having a flux function ofthe interlayer flexible wiring board 220 was replaced by an ordinaryadhesive sheet (PYRALUX LF100 produced by Du Pont) having no fluxfunction.

In the multilayer flexible wiring boards of Examples 1 to 3,interlaminar connection was secured by metal to metal bonding, and inthe temperature cycle test, there took place no disconnection and thebonded condition at the metal joint was good. Also, no rise ofinsulation resistance was observed in the insulation resistance test.Further, by cutting the outer layer single-sided wiring board into anindividual piece, the positional accuracy of the lamination was enhancedin comparison to the sheet-like lamination, resulting in an improvedyield. In the case of Comparative Example 1, however, the yield of themultilayer flexible wiring board lowered and the production and materialcosts elevated. In Comparative Example 2, metallic bond could not bemade between the two-layer posts and their receiving pads.

INDUSTRIAL APPLICABILITY

According to the present invention, high-reliability connection can bemade at the metallic joints in a wiring board laminate by use of aninterlaminar bonding agent having a function to clean the metal surface,and since there exist no connecting holes such as through-holes in thesurface of the outer layer single-sided wiring board, it is possible torealize high-density circuit wiring or high-density packaging of parts.Further, by laminating the individual pieces of wiring boards, itbecomes possible to laminate the non-defective boards alone and toproduce the multilayer flexible wiring boards in a high yield.

1. A circuit board comprising: an insulated substrate; a conductorcircuit formed on one side of said substrate; and two-layer conductorposts electrically connected to said conductor circuit; wherein each ofsaid two-layer conductor posts is formed in a hole piercing saidinsulated substrate and comprises a projecting terminal having its oneend connected to said two-layer conductor circuit and its other endprojecting from the other side of said insulated substrate, and a metalcoating layer covering the portion of said terminal that projects outfrom the other side of said insulated substrate.
 2. A circuit boardaccording to claim 1 wherein the metal coating layer is composed of atleast one metal selected from the group consisting of gold, silver,nickel, tin, lead, zinc, bismuth, antimony and copper, or an alloycontaining such metals.
 3. A circuit board comprising: an insulatedsubstrate; a conductor circuit formed on one side of said insulatedsubstrate; and two-layer conductor posts electrically connected to saidconductor circuit; wherein an adhesive layer having a function of fluxis provided on one or both sides of said insulated substrate.
 4. Acircuit board comprising: an insulated substrate; a conductor circuitformed on one side of said insulated substrate; and two-layer conductorposts electrically connected to said conductor circuit; wherein asurface coating is provided on one side of said insulated substrate,said coating covering said conductor circuit with a part thereof leftuncovered, and an adhesive layer having a flux function is provided onthe other side of said insulated substrate.
 5. A circuit board accordingto claim 1 wherein said two-layer conductor posts contain copper and ametal or copper and an alloy.
 6. A circuit board according to claim 3wherein each of said two-layer conductor posts is formed in a holepiercing said insulated substrate, and comprises a protruding terminalof which one end is connected to said conductor circuit and the otherend projects from the other side of said insulated substrate, and ametal coating layer covering the portion of said protruding terminalwhich projects from the other side of said insulated substrate.
 7. Acircuit board according to claim 6 wherein said metal coating layer ismade of at least one metal selected from the group consisting of gold,silver, nickel, tin, lead, zinc, bismuth, antimony and copper, or analloy containing such metals.
 8. A multilayer wiring board comprising alaminate of plural circuit boards including the one set forth inclaim
 1. 9. A multilayer wiring board comprising a laminate of pluralcircuit boards including the one set forth in claim
 3. 10. A multilayerwiring board comprising a laminate of plural circuit boards includingthe one set forth in claim 1 and a circuit board comprising: aninsulated substrate; a conductor circuit formed on both sides of saidinsulated substrate; a metallic layer formed covering a part of saidconductor circuit; and a surface coating covering the portion of saidconductor circuit other than said metallic layer.
 11. A multilayerwiring board comprising a laminate of plural circuit boards including afirst circuit board comprising: an insulated substrate: a conductorcircuit formed on one side of said substrate; and two-layer conductorposts electrically connected to said conductor circuit; wherein each ofsaid two-layer conductor posts is formed in a hole piercing saidinsulated substrate and comprises a protecting terminal having its oneend connected to said two-layer conductor circuit and its other endprojecting from the other side of said insulated substrate, and a metalcoating layer covering the portion of said terminal that projects outfrom the other side of said insulated substrate, a second circuit boardaccording to claim 3; and a third circuit board, comprising: aninsulated substrate; a conductor circuit formed on both sides of saidinsulated substrate; a metallic layer formed covering a part of saidconductor circuit; and a surface coating covering the portion of saidconductor circuit other than said metallic layer.
 12. A multilayerwiring board in which the circuit board set forth in claim 1 is joinedto both sides of another circuit board set forth below, and theconductor circuits of the respective circuit boards are electricallyconnected at the specified sites through said conductor posts, saidanother circuit board comprising: an insulated substrate; a conductorcircuit formed on both sides of said insulated substrate; a metalliclayer formed covering a part of said conductor circuit; and a surfacecoating covering the portion of said conductor circuit other than saidmetallic layer.
 13. A multilayer wiring board in which a first circuitboard comprising: an insulated substrate: a conductor circuit formed onone side of said insulated substrate; and two-layer conductor postselectrically connected to said conductor circuit, wherein an adhesivelayer having a function of flux is provided on one or both sides of saidinsulated substrate, is joined to both sides of another circuit boardset forth below, a circuit board set forth in claim 1 is joined to saidboth circuit boards, and the conductor circuits of the respectivecircuit boards are electrically connected at the specified positionsthrough said conductor posts, said another circuit board comprising: aninsulated substrate; a conductor circuit formed on both sides of saidinsulated substrate; a metallic layer formed covering a part of saidconductor circuit; and a surface coating covering the portion of saidconductor circuit other than said metallic layer.
 14. A multilayerwiring board according to claim 11 wherein said surface coating includesan adhesive layer.
 15. A multilayer wiring board according to claim 7having a multilayer portion comprising a laminate of plural circuitboards, and a single-layer portion to which at least one circuit boardin said multilayer portion extends therefrom.
 16. A multilayer wiringboard according to claim 15 wherein the circuit board constituting saidsingle-layer portion is a flexible circuit board.
 17. A multilayerflexible wiring board comprising (i) plural single-sided wiring boardshaving a wiring pattern formed on one side of a substrate made of aninsulating material and two-layer conductor posts made of copper and ametal or copper and an alloy, each said conductor post projecting fromsaid wiring pattern to the side of said substrate opposite from saidwiring pattern, with the substrates other than that of the outermostlayer having, on the side opposite from said conductor posts, the padsfor making connection to the conductor posts, and said wiring patternhaving no surface coating, (ii) a flexible wiring board having on atleast one side thereof the pads for connection to said conductor postsand comprising a wiring pattern with surface coating applied on theflexible portion but no surface coating applied on the multilayerportion, and (iii) an adhesive layer having a flux function whereby therespective boards are laminated integrally, wherein said conductor postsand pads are connected by a metal or an alloy through the medium of saidadhesive layer, and said wiring patterns are electrically connected. 18.A multilayer flexible wiring board according to claim 17 wherein saidflexible wiring board is a severed individual piece.
 19. A multilayerflexible printed wiring board according to claim 17 wherein the metal isat least one of gold, silver, nickel, tin, lead, zinc, bismuth, antimonyand copper.
 20. A multilayer flexible printed wiring board according toclaim 17 wherein the alloy comprises at least two of tin, lead, silver,zinc, bismuth, antimony and copper.
 21. A method of producing amultilayer flexible wiring board comprising the steps of: boring asubstrate made of an insulating material, and then forming on the boredside of said substrate the protruding two-layer conductor posts made ofcopper and a metal or copper and an alloy; forming a wiring pattern onthe side of said substrate opposite from said two-layer conductor posts;forming an adhesive layer having a flux function over the whole surfaceof each of the substrates other than that of the outermost layer on thewiring pattern side having the pads opposite from said two-layerconductor post side by lamination or printing, thereby forming asingle-sided wiring board; forming a flexible wiring board comprising awiring pattern having on at least one side thereof the pads for joiningto said two-layer conductor posts; forming an adhesive layer having aflux function on the wiring pattern side having the pads of saidflexible wiring board over the whole surface or partially thereof bylamination or printing; and heat-press bonding said two-layer conductorposts and said pads through the medium of said adhesive layer. 22.Multilayer flexible wiring boards that can be obtained from the methodset forth in claim
 21. 23. A multilayer wiring board according to claim12 wherein said surface coating includes an adhesive layer.
 24. Amultilayer wiring board according to claim 13 wherein said surfacecoating includes an adhesive layer.
 25. A multilayer flexible printedwiring board according to claim 18 wherein the metal is at least one ofgold, silver, nickel, tin, lead, zinc, bismuth, antimony and copper. 26.A multilayer flexible printed wiring board according to claim 18 whereinthe alloy comprises at least two of tin, lead, silver, zinc, bismuth,antimony and copper.
 27. A multilayer flexible printed wiring boardaccording to claim 19 wherein the alloy comprises at least two of tin,lead, silver, zinc, bismuth, antimony and copper.
 28. A circuit boardaccording to claim 4 wherein each of said two-layer conductor posts isformed in a hole piercing said insulated substrate, and comprises aprotruding terminal of which one end is connected to said conductorcircuit and the other end projects from the other side of said insulatedsubstrate, and a metal coating layer covering the portion of saidprotruding terminal which projects from the other side of said insulatedsubstrate.
 29. A multilayer wiring board comprising a laminate of pluralcircuit boards including the one set forth in claim
 2. 30. A multilayerwiring board comprising a laminate of plural circuit boards includingthe one set forth in claim
 4. 31. A multilayer wiring board in which afirst circuit board comprising: an insulated substrate; a conductorcircuit formed on one side of said insulated substrate; and two-layerconductor posts electrically connected to said conductor circuit;wherein a surface coating is provided on one side of said insulatedsubstrate, said coating covering said conductor circuit with a partthereof left uncovered, and an adhesive layer having a flux function isprovided on the other side of said insulated substrate is joined to bothsides of another circuit board set forth below, a circuit board setforth in claim 1 is joined to said both circuit boards, and theconductor circuits of the respective circuit boards are electricallyconnected at the specified positions through said conductor posts, saidanother circuit board comprising: an insulated substrate; a conductorcircuit formed on both sides of said insulated substrate; a metalliclayer formed covering a part of said conductor circuit; and a surfacecoating covering the portion of said conductor circuit other than saidmetallic layer.
 32. A multilayer wiring board comprising a laminate ofplural circuit boards including a first circuit board comprising: aninsulated substrate; a conductor circuit formed on one side of saidsubstrate; and two-layer conductor posts electrically connected to saidconductor circuit; wherein each of said two-layer conductor posts isformed in a hole piercing said insulated substrate and comprises aprojecting terminal having its one end connected to said two-layerconductor circuit and its other end projecting from the other side ofsaid insulated substrate, and a metal coating layer covering the portionof said terminal that projects out from the other side of said insulatedsubstrate, a second circuit board, according to claim 4, and a thirdcircuit board, comprising: an insulated substrate; a conductor circuitformed on both sides of said insulated substrate; a metallic layerformed covering a part of said conductor circuit; and a surface coatingcovering the portion of said conductor circuit other than said metalliclayer.